Nickel complexes in diol solvents and their use as oligomerization catalyst precursors

ABSTRACT

Ethylene is oligomerized to even numbered alpha-monoolefins principally in the C 4  to C 40  range with minimal concomittant production of high molecular weight polyethylene by reacting ethylene in a diol solvent in an oligomerization reaction zone at elevated pressure with a nickel complex catalyst composition produced by combining in the reaction zone, (a) a stable preformed complex of nickel, ethylene and hydride in a diol solvent, said complex being prepared by contacting in a diol solvent and in the presence of ethylene, a nickel salt, a base and a boron hydride transfer agent and (b) a suitable bidentate ligand; the stable nickel complex solution and the bidentate ligand being added in separate portions to the reaction zone. These stable complexes of nickel, ethylene and hydride in diol solution are considered to be novel oligomerization catalyst precursor compositions.

BACKGROUND OF THE INVENTION

This invention relates to a catalyst precursor composition and to animproved process for the production of linear alpha-olefins by catalyticoligomerization of ethylene using a catalyst obtained from the novelprecursor composition. More particularly, this invention is directed toa stable complex of nickel, ethylene and hydride in diol solvent and toan ethylene oligomerization process in diol solvent employing a nickelcomplex catalyst composition formed by adding the pre-formed stablenickel complex in diol solvent and a bidentate ligand in separateportions to the oligomerization reaction zone.

Linear monoolefins are compounds of established utility in a variety ofapplications. Terminal linear monoolefins, particularly those having 12to 20 carbon atoms per molecule, are known to be useful as intermediatesin the production of various types of detergents e.g. alcohols,ethoxylates, etc.

Several synthetic techniques have been developed for the preparation ofterminal linear monoolefins in the detergent range. One very attractivesynthetic method from the standpoint of raw material availability andcost involves oliogomerization of ethylene to higher molecular weightlinear monoolefins (even numbered alpha-monoolefins) by contact with acatalytically active nickel complex catalysts for ethyleneologomerization is prepared as the reaction product of an olefinicnickel compound, including zero-valent nickel compounds such asbis(cyclooctadiene) nickel (0) or π-allyl nickel compounds, and asuitable bidentate ligand as described in U.S. Pat. No. 3,644,564 to VanZwet et al, U.S. Pat. No. 3,647,914 to Glockner et al and U.S. 3,647,915to Bauer et al. A different and preferred class of nickel complexcatalysts can be prepared by contacting in certain polar organicsolvents in the presence of ethylene (1) a simple divalent nickel saltwhich is at least somewhat soluble in the solvent, (2) a boron hydridereducing agent and (3) a suitable bidentate ligand. The preparation ofcatalysts in this preferred class and their use in ethyleneoligomerization are described in U.S. Pat. Nos. 3,676,523, 3,686,351 and3,737,475 to R. F. Mason and U.S. Pat. No. 3,825,615 to Lutz.

In the above mentioned patents describing ethylene oligomerization withthe preferred nickel complex catalysts, it is taught that the catalystcomposition is suitably preformed outside the oligomerization reactionzone by mixing together in the presence of ethylene, the variousingredients--i.e., the nickel salt, the bidentate ligand and the boronhydride reducing agent--in the polar organic solvent. This preformedcatalytic composition in the polar organic solvent or diluent is thenadded directly to the reaction zone. In this regard, no real criticalityis attached to the order or manner in which the catalyst precursors arecombined, in the patent teachings, although it is commonly preferred tocontact the solvent, the nickel salt and the bidentate ligand in thepresence of ethylene before the boron hydride reducing agent is added tothe solution of catalyst precursors.

While the preferred nickel complex catalysts prepared according to theteachings of the aforementioned patents to R. F. Mason and to Lutzprovide an attractive means for oligomerizing ethylene to higher linearterminal olefins, in particular those in the detergent range, theresulting oligomerization processes are not devoid of problems. Onetroublesome aspect of these preferred processes is the propensity of thenickel catalysts to catalyze the formation of objectionable, polymericpolyethylene under certain conditions in the process. This polymericpolyethylene typically has a broad molecular weight range (molecularweight from a few thousand to as high as a few million) in contrast tothe desired lower molecular weight oligomer product. As produced in theoligomerization process, such polyethylene is not a usable commercialproduct and thus only serves to decrease the yield of desired oligomerproduct from the ethylene feed. Furthermore, it has an even moreobjectionable effect in that it tends to plug and foul mechanicalequipment and transfer lines in the process.

The formation of this polymeric polyethylene in the oligomerizationreaction product and its objectionable effect on downstream processingeqiupment is recognized in U.S. Pat. No. 4,020,121 to Kister et al.Specifically, this patent teaches that residual catalyst present in thehydrocarbon (oligomer) phase of the three phase oligomerization reactionproduct (the other phases being a liquid solvent phase and a gaseousethylene phase) can promote the formation of polymeric polyethylene whencatalyst, solvent and ethylene are present in the hydrocarbon phase atconditions under which part of the hydrocarbon phase is removed byflashing or distillation. According to Kister et al, this polyethyleneformation downstream of the oligomerization reaction can be avoided by astepwise product recovery process in which the hydrocarbon product phaseis subject to a scrubbing step using additional liquid reaction solventprior to the time that the catalyst-contaminated hydrocarbon productphase is subjected to depressurization for removal of ethylene.

A second troublesome source of polymeric polyethylene which is not dealtwith in the Kister et al patent is the portion of the oligomerizationprocess which is upstream of the oligomrization reaction itself, thatis, the reaction vessel (catalyst maker) used for making the preformedoligomerization catalyst and associated reactant transfer lines into thecatalyst maker and out of the catalyst maker to the oligomerizationreactor. Here, it is found, particularly when the catalyst preparationis carried out in a continuous fashion using an aliphatic diol reactionsolvent, that polymeric polyethylene tends to form and periodially plugoff the transfer line from the catalyst maker to the oligomerizationreaction zone and the transfer line used to add makeup ethylene to thecatalyst maker. Further during periods of minor upset, significantquantities of polymeric polyethylene may form in the catalyst makeritself necessitating shutdown and cleanout operations. When it forms inthe catalyst maker, the polymeric polyethylene generally consumes asubstantial quantity of the nickel catalyst component present since theresulting product is typically a solid agglomeration of polyethylene andnickel particles.

From the foregoing it can be seen that it would be highly desirable ifan alternative means could be devised for preparing and introducing thenickel complex catalyst into the oligomerization reactor which wouldminimize the formation of undesirable polymeric polyethylene and theequipment plugging and fouling problems associated therewith.

DESCRIPTION OF THE PRIOR ART

In an article entitled "The `Nickel Effect`" by Fischer et al in Angew.Chem. internat. Edit. Vol. 12, No. 12, December 1973 pp 943-1026 mentionis made on pages 951 and 952 of nickel (0) complexes with olefins,specifically tris (ethylene) nickel obtained by treating(cyclododecatriene) nickel with ethylene at 0° C. However, this tric(ethylene)-nickel complex is characterized as being rather unstable("extremely sensitive") and there is no indication in the article thatthe nickel olefin complexes could have practical value as precursors fornickel complex oligomerization catalysts.

SUMMARY OF THE INVENTION

A simple and economic technique has now been found to minimize polymericpolyethylene formation in ethylene oligomerization reaction systemsemploying aliphatic diol reaction solvents containing nickel complexcatalysts prepared by combining, in the presence of ethylene, a simpledivalent nickel salt, a boron hydride transfer agent, a base and asuitable bidentate ligand. In this improved process, ethylene isoligomerized to even numbered alpha-monoolefins by reacting ethylene ina diol solvent in an oligomerization reaction zone at elevated pressurewith a nickel complex catalyst composition produced by combining in thereaction zone (a) a stable preformed complex of nickel, ethylene andboron hydride in a diol solvent, said complex being prepared bycontacting in a diol solvent and in the presence of ethylene, (1) asimple divalent nickel salt, (2) a base and (3) a boron hydride transferagent and (b) a suitable bidentate ligand; the stable preformed complexof nickel in diol solvent and the bidentate ligand being added inseparate portions to the reaction zone. By performing the stable complexof nickel, ethylene and hydride in diol solvent and adding it to theoligomerization reaction zone separately from the bidentate ligand, ithas been found not only that the problem of polymeric polyethyleneformation which plagued the catalyst preparation phases of the prior artoligomerization processes, discussed above, have been solved but,additionally and quite surprisingly, the catalyst utilization in thereaction zone is also enhanced since oligomer product can be produced ata rate equivalent to the prior art processes with a significantreduction in the consumption of nickel salt and boron hydride. Thisreduced consumption of nickel salt and boron hydride reducing agent hasa further benefit in that there is less tendency for fouling when thediol solvent is recovered from the reaction product using a recoverysystem such as that described in U.S. Pat. No. 4,020,121 to Kister etal.

Accordingly, the present invention provides an improved process for theoligomerization of ethylene to linear alpha olefins which comprisesreacting ethylene in a diol solvent in an oligomerization reaction zoneat elevated pressure with a nickel complex catalyst composition producedby combining in the reaction zone (a) a stable preformed complex ofnickel, ethylene and hydride in a diol solvent, said complex beingprepared by contacting in the diol solvent and in the presence ofethylene (1) a simple divalent nickel salt (2) a base and (3) a boronhydride transfer agent and (b) a suitable bidentate ligand; the stablenickel complex in diol solvent and the bidentate ligand being added inseparate portions to the reaction zone. Also within the scope of thepresent invention is the stable preformed complex solution of nickel,ethylene and hydride in diol solvent which functions as a catalystprecursor for the nickel complex catalyst composition employed in theoligomerization process of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The improved process of the invention and the novel nickel complexcatalyst precursor solutions employed therein are broadly applicable toany ethylene oligomerization process employing a diol reaction solventand a nickel complex catalyst derived from combining a simple divalentnickel salt, a boron hydride reducing agent and bidentate ligand in thepresence of ethylene and the diol solvent. In particular the improvementaccording to the invention is suitably employed in ethyleneoligomerization processes utilizing an organo-phosphine bidentate ligandsuch as those described in U.S. Pat. Nos. 3,676,523, 3,686,351 and3,737,475 all to R. F. Mason and U.S. Pat. No. 3,825,615 to Lutz. As istaught in these patent disclosures, which are herewith incorporated byreference, the bidentate chelating ligand is suitably anorganophosphorine having a tertiary organophosphorus moiety with asuitable functional group substituted on a carbon atom attached morethan two carbon atoms from the phosphorus atom of the organophosphorusmoiety. Representative ligands of this type are compounds of the generalformula: ##STR1## wherein R, independently, is a monovalent organogroup, R' a monovalent hydrocarbyl group, X is carboxymethyl orcarboxyethyl; Y is hydroxymethyl, mercaptomethyl, hydrocarbyl of up to10 carbon atoms or hydrycarbyloxy carbonyl of up to 10 carbon atoms; Ais hydrogen or an aromatic group of up to 10 carbon atoms; M is hydrogenor an alkali metal, preferably sodium or potassium; x and y are zero,one or two and the sum of x and y is two, with the proviso that when xis two the R groups may together with the phosphorus atom form a mono-or bicyclic heterocyclic phosphine having from 5 to 7 carbon atoms ineach ring thereof. Preferred ligands are o-dihydrocarbylphosphinobenzoicacid or its alkali metal salt as described in U.S. Pat. No. 3,676,523with o-diphenylphosphinobenzoic acid being most preferred. Anotherpreferred ligand, described in U.S. Pat. No. 3,825,615, isdicyclohexylphosphinopropionic acid or its alkali metal salt.

The nickel salt employed to make up the stable nickel complex catalystprecursor and the oligomerization catalyst is suitably any simpledivalent nickel salt which is sufficiently soluble in the diol solventto provide a catalytically effective concentration of nickel complexcatalyst. By the terms "simple divalent" nickel salt is meant a nickelatom having a formal valence of +2 and bonded through ionic orelectrovalent linkages to two singly charged anionic groups (e.g.,halides) or to one doubly charged anionic group (e.g., carbonate) andnot complexed with or coordinated to any other additional molecular orionic species with the exception of waters of hydration. Simple divalentnickel salts therefore do not encompass complex divalent nickel saltswhich are bonded to one or two anionic groups and additionally complexedor coordinated to neutral chelating ligands or groups such as carbonmonoxide and phosphines. However, simple divalent nickel salts are meantto include nickel salts containing water of hydration in addition to oneor two anionic groups. Suitably, the simple divalent nickel saltemployed to prepare the catalyst precursor and oligomerization catalystwill have a solubility of at least 0.0005 mole per liter (0.0005 M) inthe diol solvsent. A solubility in the diol solvent used to prepare thenickel catalyst precursor is preferably at least 0.001 mole of nickelsalt per liter (0.001 M) and most preferably at least 0.005 mole ofnickel salt per liter of diol solvent (0.005 M). In this regard suitabledivalent nickel salts include inorganic nickel salts as well as organicdivalent nickel salts. Illustrative inorganic nickel salts are nickelhalides such as nickel chloride, nickel bromide and nickel iodide,nickel carbonate, nickel chlorate, and nickel nitrate. Illustrativeorganic divalent nickel salts are nickel salts of carboxylic acids suchas nickel alkanoates of up to 10 carbon atoms, preferably of up to 6carbon atoms, e.g., nickel formate, nickel acetate, nickel propionate,nickel hexanoate and the like; nickel oxalate, nickel benzoate andnickel naphthenate. Other suitable organic salts include nickelbenzenesulfonate, nickel citrate, nickel dimethylglyoxime and nickelacetylacetonate. Nickel halides, especially nickel chloride, and nickelalkanoates, especially nickel acetate, in part because of theiravailability at low cost and solubility in diol solvents, are preferrednickel salts.

As a general matter, any boron hydride salt reducing agent of reasonablepurity can be suitably employed to prepare the stable nickle complexcatalyst precursor of the invention. Specific examples include alkalimetal borohydrides such as sodium borohydrides, potassium borohydrideand lithium borohydride; alkali metal alkoxyborohydrides wherein eachalkoxy has 1-4 carbon atoms, such as sodium trimethoxyborohydride andpotassium tripropoxyborohydride and tetraalkylammonium borohydrideswherein each alkyl has 1-4 carbon atoms, such as tetraethylammoniumborohydride. Largely because of commercial availability, alkali metalborohydrides are preferred and especially preferred is sodiumborohydride.

The diol solvent used both to prepare the stable nickel complex catalystprecursor of the invention and as the reaction solvent in the improvedoligomerizations process of the invention is an aliphatic diol of 2 to 7carbon atoms. While different aliphatic diol solvents may be employed inthe preparation of the catalyst precursor and in the oligomerizationreaction itself, it is preferable to use the same diol solvent in bothoperations. In this regard, suitable aliphatic diols include vicinalalkane diols such as ethylene glycol, propylene glycol,2-methyl-1,2-propane-diol, 1,2-butanediol and 2,3-butanediol andalpha-omega alkane diols such as 1,4-butanediol, 1,5-pentanediol,1,6-hexanediol and 1,7-heptandiol. Alpha-omega alkane diols of 4 to 6carbon atoms are preferred solvents with 1,4-butanediol beingparticularly preferred. In some cases it may be desirable to employmixtures of the abovementioned alkane diols as the solvent source forthe catalyst precursor preparation and/or the oligomerization reaction.

The base employed in combination with the nickel salt, the boron hydridetransfser agent and ethylene to make up the stable nickel complexcatalyst precursor in diol solvent is suitably an alkali metal oralkaline earth metal hydroxide. In this regard it is preferable toemploy alkali metal hydroxides with sodium or potassium hydroxide beingmost preferred. Typically, the base is used aes about a 0.1 to 3.0 Msolution in water.

To effect the improvement according to the invention, the stable nickelcomplex catalyst precursor in diol solvent is prepared in a reactionzone apart from the oligomerization reaction zone and then addedseparately to the reaction zone along with the desired bidentate ligand.This catalyst precursor preparation step may be carried out batchwise orcontinuously and is suitably accomplished by contacting or mixingtogether in the aliphatic diol solvent and in the presence of ethylenethe nickel salt, the boron hydride reducing agent and the base. Theorder in which the nickel component, boron hydride component and baseare combined in the presence of ethylene and the diol solvent in thereaction zone is important. For best results, the nickel salt in diolsolution and the base in aqueous solution are added to the diol solventand to this mixture, the aqueous boron hydride containing some base as astabilizing agent is added. As noted above, it is essential that thenickel complex catalyst precursor be prepared in the presence ofethylene. Suitably, the ethylene pressure and contacting conditionsshould be sufficient to saturate the catalyst precursor solution withethylene. Typically, the ethylene pressures employed may be in the rangeof from 10 to 5000 psig or higher with pressures in the range of fromabout 500 to about 2000 psig being preferred. The molar ratio of nickelto boron hydride reducing agent in the preparation of the catalystprecursor suitably ranges from about 0.2:1.0 to about 2.0:1.0 with molarratios of about 0.5:1.0 to 1.0:1.0 being preferred. Similarly, the molarratio of nickel to the base component is suitably in the range of about0.33:1.0 to about 10.0:1.0 with ratios of about 0.5:1.0 to 40:1.0 beingpreferred. In preparing the catalyst precursor sufficient diol solventis used to provide a nickel complex solution containing from about0.0002 to 0.2% by weight nickel. This diol solvent may be added togetherwith the nickel salt as a diol solution of the nickel salt oralternatively, all or a portion of the diol solvent may be added as aseparate charge to the catalyst precursor preparation reaction zone.

The reaction temperatures required to prepare the stable nickel complexin diol solvent in the catalyst precursor preparation reaction zonesuitably range between about 0° and about 100° C. with temperatures inthe range of 20° to 50° C. being preferred. In any case, reactiontemperatures above about 100° C. should be avoided to minimizedecomposition of the nickel complex catalyst precursor and if theresulting diol solution of nickel complex catalyst precursor is to bestored prior to use in the oligomerization reaction, its temperatureshould be maintained below about 40° C. to insurance optimum stability.The reaction time required to complete the catalyst precursor complexformation typically is less than one minute. Upon completion of thenickel complex forming reaction in the diol solvent, it is essential tothe stability of the nickel complex that the solvent containing thenickel complex catalyst precursor be maintained under sufficientethylene pressure to insure solution saturation with ethylene duringtransport and storage of the catalyst precursor solution.

In a preferred aspect of the invention, the nickel complex catalystprecursor solution in diol solvent is prepared continuously by chargingthe boron hydride reducing agent e.g., sodium borohydride, and the basee.g., potassium hydroxide, as a premixed aqueous solution to an agitatedreaction vessel maintained at under ethylene °C. pressure e.g., 1500psig, simultaneous with the addition of a solution of the simpledivalent nickel salt e.g., nickel chloride, in the aliphatic diolsolvent e.g., 1,4-butanediol. In this preferred embodiment, the flowrates of the mixed aqueous stream containing boron hydride and base andthe aliphatic diol stream containing nickel salt are controlled toafford a nickel to boron hydride molar ratio of about 0.6:1.0 and anickel to base molar ratio of about 2.0:1.0. If desired, all of thealiphatic diol solvent can be added with the nickel salt oralternatively a portion of the diol solvent recovered from theoligomerization reaction in accordance with the process of U.S. Pat. No.4,020,121 may be added as a separate stream to afford the desiredconcentration of nickel complex catalyst precursor in the diol solvent.For this preferred embodiment, sufficient diol solvent is added on acontinuous basis to give a nickel complex concentration, based onnickel, in the diol solvent of about 0.015% by weight. Further, thereaction vessel size and the flow rates of reactants are selected togive the reactants at least a 1 minute residence time in the reactionvessel prior to withdrawal and changing to the oligomerization reactionzone.

The structure of the nickel complex catalyst precursor in diol solventprepared as indicated above has not been determined. Based on knownbehavior of nickel with cyclooctadiene and other olefins, it is possiblethat the structure is a complex in which two molecules of ethylene arecombined with one molecule of chemically reduced nickel (nickel bondedto at least one hydrogen atom). When formed from nickel chloride,potassium hydroxide, sodium borohydride and ethylene, a 1,4-butane diolsolution of this nickel complex has the following properties: (a) it isa clear colorless solution, (b) it is only stable under ethylenepressure--when depressured it darkens rapidly with deposition of nickelmetal, (c) it reduces aqueous iodine solution, (d) it is decomposed atacidic pH and (e) in aliphatic diol/hydrocarbon systems, it distributesstrongly in favor of the aliphatic diol. As indicated previously, thenickel complex catalyst precursor solutions of the invention arerelatively stable and, even at temperatures of 80°-90° C., the half lifeof the catalyst precursor complex is sufficient (about 10 minutes) thatwhen combined with an appropriate bidentate ligand the rate of formationof oligomerization catalyst at these temperatures clearly exceeds therate catalyst precursor decomposition. This finding of catalystprecursor stability is critical to the operability of the process of theinvention because the oligomerization reaction is typically carried outat such elevated temperatures.

In the improved process of the invention, ethylene is oligomerized byreacting ethylene in an aliphatic diol solvent, in an oligomerizationreaction zone at elevated pressure with a nickel complex catalyst formedby combining in the reaction zone (a) an aliphatic diol solution of anickel complex catalyst precursor prepared as described above and (b) asuitable bidentate ligand; the nickel complex catalyst precursorsolution and the bidentate ligand being added in separate portions tothe reaction zone. The reaction temperatures employed to oligomerizeethylene with the improvement according to the invention suitably rangebetween about 50° and about 150° C. with temperatures in the range offrom about 80° to 120° C. being preferred. The pressure in theoligomerization reaction zone must be at least sufficient to maintainthe reaction mixture substantially in the liquid phase although excessethylene will be present in the vapor phase. In this regard, totalreaction zone pressures of from about 300 to 5000 psig are suitablyemployed. Of greater importance to the operability of theoligomerization process is the partial pressure of ethylene in thereaction zone since ethylene partial pressure is a primary factor inmaintaining the desired ethylene concentration in the diol solvent phasewhere the oligomerization reaction takes place. Satisfactory reactionrates can be obtained in the oligomerization process of the inventionswith ethylene partial pressures in the reaction zone of from about 400to 2500 psig. Preferably, the ethylene partial pressure in the reactionzone is maintained at between about 1000 and 2500 psig. The improvedoligomerization catalyst employed in the process of the invention isformed in the oligomerization reaction zone by adding the diol solutionof nickel complex catalyst precursor and bidentate ligand in a molarratio of catalyst precursor (based on nickel) to bidentate ligand offrom about 0.5:1.0 to about 5.0:1.0, with preferred ratios being in therange of about 1.0:1.0 to 2.5:1.0. When its preferred phosphine ligandsof U.S. Pat. No. 3,676,523 to Mason and U.S. Pat. No. 3,825,615 to Lutzi.e., o-diphenylphosphinobenzoic acid and dicyclohexylphosphinopropionicacid are employed, exceptional catalyst activity is obtained with nickelcomplex catalyst precursor to phosphine ligand molar ratios of about1.5:1.0. In contrast, with the preformed nickel-ligand catalyst systemsof the prior art, such as the catalysts disclosed in the aforementionedU.S. patents, it is necessary to maintain the molar ratio of nickel toligand in the range of about 2.0:1.0 to obtain similar oligomerizationcatalyst activity. This reduced consumption of nickel and boron hydridereducing agent obtained with the present process provides a clearadvantage for the present invention over the prior art both from thestandpoint of catalyst cost and ease of product recovery. With theoligomerization process of the invention, the nickel complex catalystprecursor and bidentate ligand are added to the reaction zone to providea concentation of catalyst (calculated as parts per million of ligand inthe diol reaction solvent) of at least 10 ppm, with catalystconcentrations of from about 50 to about 1000 ppm being preferred andconcentrations of from 75 to 125 ppm being most preferred. In thisregard, the amount of aliphatic diol reaction solvent present in thereaction zone may suitably range up to 30 liters per mole of ethylenewith from about 0.1 to 1.0 liters of solvent per mole of ethylene beingpreferred.

The improved oligomerization process of the invention may be carried outin batch or continuous manner. In a suitable batch operation thereactant ethylene and catalyst components i.e., nickel complex catalystprecursor in diol solvent and bidentate ligand, are added as separatecomponents to an agitated reaction vessel, with the ethylene partialpressure in the vessel being maintained at a preselected pressurethroughout the reaction period i.e. 10 to 1000 minutes. Since ethyleneis consumed during the oligomerization reaction, additional ethylene isadded on an incremental basis during the batch reaction to maintain thedesired ethylene partial pressure. Further, if desired, part of the diolreaction solvent may be added as a separate component to the reactionvessel. As ethylene is oligomerized during the course of the batchreaction, a separate hydrocarbon or oligomer liquid phase forms in thereaction vessel such that the reaction product consists of a three phasemixture i.e., an ethylene vapor phase, a liquid diol solvent phaecontaining dissolved catalyst and a liquid hydrocarbon or oligomerproduct phase containing dissolved ethylene, as well as small amounts ofdiol solvent and catalyst.

Preferably, the improved oligomerization process of the invention iscarried out in a continuous fashion. With certain modifications, thispreferred mode of conducting the process of the invention may be carriedout using the reaction system described in U.S. Pat. No. 4,020,121 toKister et al.; the modifications being the use of the catalystpreparation zone to make the stable nickel complex catalyst precursorsolution in diol solvent and the separate addition of this catalystprecursor solution and the bidentate ligand to the reaction loop. Inparticular, a preferred reaction system for continuous oligomerizationof ethylene in accordance with the invention includes two or more (mostpreferably three) time tank reactors arranged in series with appropriatepumping for continuous circulation of the reaction mixture through thereactors. Each time tank reactor is followed by a heat exchanger throughone side of which a heat exchange fluid is circulated to remove heat ofreaction and thus to maintain the appropriate temperatures in thereactors. The inlet lines for the two components making up the catalysti.e., the nickel complex catalyst precursor in diol solution and thebidentate ligand, are separated so that the nickel complex catalystprecursor solution and the bidentate ligand are added into the reactionloop upstream of two different reaction stages. Preferably, thebidentate ligand is added to the circulating reaction mixture at a pointupstream of the point at which the nickel complex solution is added. Forexample, in the three stage reaction system the bidentate ligand isadded from storage through an inlet port located immeditely upstream ofthe first or second time tank reactor stage whereas the diol solution ofnickel complex catalyst precursor is made as described above in acontinuous catalyst precursor prepartion vessel and added to thecirculating reaction mixture in the oligomerization reaction loopimmediately upstream of the second or third reaction stage,respectively. The complete reaction requirement of ethylene may be addedcontinuously along with the diol solution of nickel complex catalystprecursor i.e., via ethylene charge to the catalyst precursorpreparation vessel, or a portion of the ethylene may be added as aseparate stream to the reaction loop. Preferably, only a small fractionof the ethylene feed is added at catalyst precursor preparation vessel(to maintain precursor stability), and most of the ethylene is addedseparately to the oligomerization reaction loop. Similarly, all of thealiphatic diol reaction solvent may be added with the nickel complexcatalyst precursor or a portion of the solvent may be added as aseparate stream to the reaction loop. The time tank reactors andreaction loop are sized and the flows of catalyst components arecontrolled to give the circulating reaction mixture a sufficientresidence time e.g., 5 to 50 minutes, based on reactor volume and liquidcirculation rate, so that a portion of the finished oligomer product canbe withdrawn from the circulating reaction mixture leaving the thirdreaction stage and passed to product recovery. Typically the portion ofthe reaction mixture split off and sent to product recovery will be nomore than 10%w of the total circulating reaction mixture in the reactionloop. This reaction product is also comprised of the three phasesmentioned above for the batch reaction process.

Regardless of whether the oligomerization is carried out batchwise orcontinuously, the oligomer product, diol solvent, active catalyst andunreacted ethylene are suitably recovered from the reaction loopeffluent using the recovery technique disclosed in U.S. Pat. No.4,020,121 to Kister et al as modified by the aqueous acid hydrolysis andextraction procedure described in U.S. Pat. application Ser. No. 50,904filed June 21, 1979 (common assignee). The disclosures of theaforementioned U.S. Pat. application and U.S. Pat. No. 4,020,121 areherewith incorporated by reference for their teaching of a preferredproduct recovery process. In summary terms, the combined teachings ofthese two references provide for a stepwise product and solvent recoveryscheme wherein (a) the reaction effluent is fed to a gas liquidseparation zone wherein gaseous ethylene is separated from the liquidproduct at temperatures and pressures approximating the reaction zoneconditions; (b) the separated liquid product comprising the liquidsolvent phase and hydrocarbon phase is passed to one or moreliquid-liquid separation zones in which a substantial portion of liquiddiol solvent and catalyst complex are removed to afford a liquidhydrocarbon product phase containing dissolved ethylene and a smallamount of solvent and catalyst complex; (c) the phase separated liquidhydrocarbon product is scrubbed with purified or fresh diol reactionsolvent under sufficient pressure to avoid flashing of dissolvedethylene, said solvent serving to remove residual active catalyst fromthe hydrocarbon phase; (d) the catalyst-free, hydrocarbon product ispassed to a deethenizer wherein dissolved ethylene is flashed off atreduced pressure to afford a deethenized ydrocarbon product containingminor amounts of diol solvent; (e) the deethenized product is washedwith water to remove residual diol solvent thereby affording a liquidoligomer product essentially free of solvent, catalyst and ethylene; and(f) the water washed liquid oligomer product is contacted withsufficient aqueous acid (pH below about 5) at elevated temperatures tohydrolyze and extract any diol solvent decomposition products which formduring oligomerization and/or aforementioned recovery steps. Applyingthis recovery process to the improved oligomerization process of theinvention, the gaseous and entrained ethylene recovered in steps (a) and(d) is suitably passed back to the catalyst precursor preparation vesseland/or the reaction loop and the diol solvent containing active catalystrecovered in steps (b) and (c) is suitably recycled to the catalystprecursor preparation vessel and/or directly to the reaction loop. Thepurified oligomer product obtained using this recovery technique issuitably passed to a product work-up system typically consisting of aseries of fractionation columns for recovery of the various oligomerfractions. The alpha olefins products may be recovered as individualcarbon number alpha olefins, e.g., C₁₀, C₁₂, C₁₄, etc., or as mixturesof even carbon numbers.

The effectiveness of the improved oligomerization process of theinvention is demonstrated by the following illustrative embodiments.

ILLUSTRATIVE EMBODIMENT I

The stability of the nickel complex catalyst precursor in an aliphaticdiol solution ws demonstrated by heating representative samples of1,4-butanediol solutions of a nickel complex catalyst precursor forextended periods of time under ethylene pressure and analyzingintermittantly for loss of nickel complex. The 1,4-butanediol solutionsof the nickel complex catalyst precursor were prepared by combining 2.6to 11.9 millimoles of nickel chloride hexahydrate (NiCl₂.6H₂ O), 1000 gof 1,4-butanediol, 3.4 to 15.5 millimoles of sodium borohydride and 1.14to 5.2 millimoles of portassium hydroxide in a 4 liter Magnadriveautoclave maintained under an ethylene pressure of 700 psig at 25° C.After additions of the catalyst precursor components to the autoclave,the 1,4-butandiol solution was agitated under ethylene pressure at 24°C. for a period of time and then heated slowly to about 87° C. (typicaloligomerization) reaction temperature) with a corresponding increase inthe ethylene partial pressure. To determine the stability of the nickelcomplex catalyst precursor solution at various temperatures, samples ofthe solution were periodically withdrawn and tested for total reducingpower by iodimetry.

The total reducing power (TRP) of samples of catalyst precursor in diolwas measured as follows: A weighed sample of catalyst precursor solutionwas taken into a known amount of aqueous tri-iodide solution under aninert atmosphere. Some of the iodine was reduced by the catalystprecursor. The remaining iodine was measured by titration with standardsodium thiosulfate solution. The total reducing power was calculated asthe difference between this titration and the titration of a blank whichcontained iodine solution but no sample.

Experimental data suggest that one equivalent of reducing power ismeasured for each mole of nickel which is present in the diol solutionas catalyst precursor. Because of residual borohydride decompositionwhich occurs in the first 15-20 minutes of the tests the base level fordetermining actual stability of the nickel complex was taken from thesample withdrawn at 30 minutes after the catalyst precursor componentshad been combined. The results of the stability tests including relevanttest conditions are given below in Table I.

                  TABLE I                                                         ______________________________________                                                                      Tem-                                            Catalyst Precursor Solution                                                                        Ethylene per-   Half                                     Ni conc                                                                              KOH:Ni    NaBH.sub.4 :Nickel                                                                        Pressure                                                                             ature                                                                              life                                 (μm/g)                                                                            Mole Ratio                                                                              Mole Ratio  (psig) °C.                                                                         (min.)                               ______________________________________                                        11.9   0.44:1    1.3:1        700   25   >250                                 11.9   0.44:1    1.3:1       1300   87     22                                 2.63   0.44:1    1.3:1        700   25   >300                                 2.63   0.44:1    1.3:1       1300   87     15                                 ______________________________________                                    

ILLUSTRATIVE EMBODIMENT II

To demonstrate the improved process of the invention, ethylene wasoligomerized in a continuous reaction system using a preformednickel-ligand complex catalyst prepared according to the prior art inthe first part of the test followed by ethylene oligomerization with acatalyst composition made using the same catalyst components butprepared according to the invention by adding the diol solution ofnickel complex catalyst precursor and ligand to the oligomerizationreaction zone as separate streams. The reaction system employed for thistest program included an agitated vessel which served as the catalystpreparation vessel for the preformed catalyst case and as the catalystprecursor preparation vessel in the improvement according to theinvention. The oligomerization reaction was carried out in both cases ina reaction loop comprised of three time tank reactors arranged in serieswith a pump and recycle line for continuous circulation of the reactionmixture through the reactors, and with reactor intercoolers for controlof temperature. In both cases, the excess ethylene, oligomer product,solvent and catalyst were recovered using the process described in U.S.Pat. No. 4,020,121 to Kister et al. In part of the test run carried outaccording to the prior art, the catalyst was prepared by combining thenickel salt component and the ligand component, both in diol solvent, ina line leading to the catalyst preparation vessel whereas the boronhydride component and base component were separately added to thecatalyst preparation vessel as a premixed aqueous solution. Ethylenerequired for the preformed catalyst was added to the catalystpreparation vessel via a separate line along with additional diolsolvent. All of the oligomerization reaction requirement of diol solventwas added to the reaction system via the catalyst preparation vessel,but the bulk of the ethylene was added directly to the reactor loop.After start up, the fresh diol solvent added directly to the catalystpreparation vessel was replaced with diol solvent recovered from theoligomerization reaction product. Further, part of the ethylenerecovered from the reaction product was also recycled to the catalystpreparation vessel after start up of the oligomerization reaction, andthe rest of the recycle ethylene was added directly to the reactor loop.In the oligomerization using the preformed nickel complex catalyst ofthe prior art, the catalyst in diol solution and its contained ethylenewere added to the oligomerization reaction loop at a point immediatelyupstream of the third reactor stage. For the improvement according theinvention, the oligomerization reaction system was modified so that theligand component was added in diol solution as a separate stream to theoligomerization reaction loop; the point of addition being immediatelyupstream of the second reactor stage. Thus, in the improvement accordingto the invention, the catalyst preparation vessel was used only to makethe nickel complex catalyst precursor by combining the simple nickelsalt, the boron hydride reducing agent and the base in the presence ofethylene and diol solvent in the manner described above for making thepreformed catalyst; the only difference being that the ligand componentwas diverted directly to the reaction zone. As in the case of thepreformed catalyst, the nickel complex catalyst precursor solution wasadded to the oligomerization reaction loop at a point upstream of thethird reactor stage. In both cases, oligomerization reaction product waswithdrawn from the reactor loop at a point downstream from the thirdreactor stage. This product was made up of a liquid solvent phasecontaining nickel complex catalyst and saturated with ethylene, a liquidhydrocarbon or oligomer phase dispersed in the solvent phase andcontaining dissolved ethylene, solvent and catalyst, and a gaseousethylene phase.

The test program (both phases) was carried out using 1,4-butanediol asthe catalyst preparation and oligomerization solvent, nickel chloridehexahydrate (NiCl₂.6H₂ O) as the nickel salt catalyst component, sodiumborohydride as the boron hydride catalyst component, potassium hydroxideas the base catalyst component and o-diphenylphosphinobenzoic acid asthe ligand catalyst component. The nickel salt was employed as an 8%wsolution in 1,4-butanediol while the ligand was added as a 3.5%wsolution in the diol solvent. The sodium borohydride and potassiumhydroxide were added as a mixed aqueous solution, 9.4%w and 4.2%w,respectively.

The specific reaction conditions common to both the preformed catalystcase and the improvement according to the invention were as follows:

    ______________________________________                                        a.  Turnover time in reaction loop                                                                       15 minutes                                         b.  Solvent/oligomer weight ratio                                                                        10/1                                               c.  Ethylene pressure in reaction loop                                                                   1500 psig                                          d.  Oligomerization reaction temperature                                                                 190-205° F.                                 e.  Weight ratio of recirculated                                                  reaction mixture to product                                                   drawn off reactor loop 40:1                                               f.  Mole ratio of nickel component                                                to boron hydride component in                                                 catalyst or catalyst precursor                                                                       0.5:1.0 to 0.7:1.0                                 g.  Mole ratio of nickel component to                                             base component in catalyst or                                                 catalyst precursor     1.5:1.0 to 2.1:1.0                                 h.  Concentration of ligand in                                                    reaction solvent       90 to 100 wt. ppm                                  ______________________________________                                    

Because of differences in the preparation of the catalyst in thepreformed catalyst case versus the process of the invention there weresome inherent differences in the two reaction systems. Further, certainreaction conditions were modified during the course of the continuousoligomerization according to the invention as compared to those employedin the preformed catalyst case in response to the improved properties ofthe catalyst prepared according to the invention. In particular, it wasfound that the quantity of nickel complex catalyst precursor could besubstantially reduced relative to the quantity of ligand charged withoutany significant reduction in catalytic activity over that achieved withthe preformed catalyst. These differences or modified reactionconditions are noted below with the reduced nickel complex consumptionin the improvement according to the invention being expressed as areduction in the mole ratio of nickel component to ligand componentcharged to the reaction zone:

    ______________________________________                                        (1) Preformed catalyst case                                                   (a)   temperature in catalyst                                                       preparation vessel 100° F. -(b) ethylene pressure in cata-             lyst preparation vessel                                                                          1500 psig                                            (c)   mole ratio of nickel compo-                                                   nent to ligand component                                                      in catalyst        2.1:1.0                                              (d)   catalyst concentration in                                                     oligomerization reaction                                                      loop based on nickel input                                                                       0.014% w.                                            (2) Improvement according to invention                                        (a)   temperature in catalyst                                                       precursor preparation vessel                                                                     100°F.                                        (b)   ethylene pressure in catalyst                                                 precursor preparation vessel                                                                     1500 psig                                            (c)   mole ratio of nickel component                                                to ligand component in cata-                                                  lyst (range over test period)                                                                    1.4:1.0 to 2.1:1.0                                   (d)   catalyst reaction loop                                                        based on nickel input (range                                                  over test period)  0.009% w. to 0.014% w.                               ______________________________________                                    

The results of the continuous oligomerication reaction test program arerecorded below in Table II. In this table, K factor is used inaccordance with its definition given in the prior art e.g. see U.S. Pat.No. 4,020,121, to designate the distribution of oligomers by carbonnumber obtained in the oligomerization reaction product. This productdistribution constant of K factor is a mathematical expression havingthe following definition: ##EQU1##

                  TABLE II                                                        ______________________________________                                        Hours                Approximate                                              into  Mole Ratio     Rates of Oligomer                                        Test  of Nickel Component                                                                          Product Formation                                        Run   to Ligand Component                                                                          (lb/hr)       K factor                                   ______________________________________                                        Preformed catalyst case                                                       10    2.1            49,000        0.77-0.776                                 Improvement according to Invention                                            11    2.1            52,500        0.783                                      13    2.1            53,200        0.788                                      15.5  1.75           56,700        0.788                                      20.5  1.65           55,300        0.776                                      32    1.65           53,200        0.778                                      34    1.40           56,000        0.754                                      60    1.40           53,200        0.750                                      75    1.40           50,400        0.754                                      ______________________________________                                         Throughout the test run period using the process according to the     invention the reaction loop remained consistently free of black or grey     color which characterizes the presence of metallic nickel in the system.     Further, as the test run according to the invention progressed, the     reactor loop samples contained progressively less floc (presumably     polymer) until at the end of the test the samples were essentially free of     floc. The presence of black or grey color (metallic nickel) and floc in     the reaction loop are inherent problems characterizing the use of the     preformed catalyst of the prior art.

What is claimed is:
 1. A process of oligomerizing ethylene to linear,alpha olefins which comprises reacting ethylene in an aliphatic diolsolvent in an oligomerization reaction zone at elevated pressure with anickel complex catalyst composition produced by combining in thereaction zone (a) a stable preformed complex of nickel, ethylene andhydride in an aliphatic diol solvent, said complex being prepared bycontacting in the diol solvent and in the presence of ethylene (1) asimple divalent nickel salt (2) a base and (3) a boron hydride transferagent and (b) a suitable bidentate ligand; the stable nickel complex indiol solvent and the bidentate ligand being added in separate portionsto the reaction zone.
 2. The process according to claim 1, wherein thebidentate ligand employed is an organo-phosphine having a tertiaryorganophosphorus moiety with a suitable functional group substituted ona carbon atom attached more than two carbon atoms from the phosphorusatom of the organophosphorus moiety.
 3. The process according to claim2, wherein the organophosphorus bidentate ligand is selected fromcompounds having the general formula: ##STR2## wherein R, independently,is a monovalent organo group, R' a monovalent hydrocarbyl group, X iscarboxymethyl or carboxyethyl; Y is hydroxymethyl, mercaptomethyl,hydrocarbyl of up to 10 carbon atoms or hydrycarbyloxycarbonyl of up to10 carbon atoms; A is hydrogen or an aromatic group of up to 10 carbonatoms; M is hydrogen or an alkali metal; x and y are zero, one or twoand the sum of x and y is two, with the provison that when x is two theR groups may together with the phosphorus atom form a mono- or bicyclicheterocyclic phosphine having from 5 to 7 carbon atoms in each ringthereof.
 4. The process according to claim 3, wherein theorganophosphine bidentate ligand is an o-dihydricarbylphosphinobenzoicacid.
 5. The process according to claim 3, wherein the organophosphinebidentate ligand is selected from the class consisting ofo-diphenylphosphinobenzoic acid and dicyclohexylphosphinopropionic acid.6. The process according to claim 3, wherein the simple divalent nickelsalt has a solubility in the aliphatic diol solvent used to prepare thestable preformed complex of nickel, ethylene and hydride of at least0.0005 mole per liter of solvent.
 7. The process according to claim 6,wherein the simple divalent nickel salt is selected from the classconsisting of nickel halides and nickel alkanoates; the base is selectedfrom the class consisting of alkali metal hydroxdes and alkaline earthmeatal hydroxides; the boron hydride transfer agent is selected from theclass consisting of alkali metal borohydrides, alkali metalalkoxyborohydrides and tetraalkylammonium borohydrides and the aliphaticdiol solvent employed to prepare the stable preformed complex of nickel,ethylene and hydride is an aliphtic diol of 2 to 7 carbon atoms selectedfrom the class consisting of vicinal alkane diols and alpha,omega-alkane diols.
 8. The process according to claim 7, wherein thestable preformed complex of nickel, ethylene and hydride in aliphaticdiol solvent is prepared under an ethylene pressure of about 10 psig toabout 5000 psig at a temperature of about 0° to about 100° C. and theoligomerization reaction as carried out under an ethylene pressure inthe oligomerization reaction zone of about 10 to about 5000 psig at anoligomerization reaction zone temperature of about 50° to about 150° C.9. The process according to claim 8, wherin the aliphatic diol solventused to prepare the stable preformed complex solution of nickel,ethylene and hydride and the aliphatic diol solvent used in theoligomerization reaction zone are the same.
 10. The process according toclaim 9, wherein the stable preformed complex of nickel, ethylene andhydride is prepared using a nickel salt to boron hydride molor ratio offrom about 0.2:1.0 to about 2.0:1.0 and a nickel salt to base molarratio of about 0.33:1.0 to about 10.0:1.0 using sufficient aliphaticdiol solvent to afford a nickel complex solution containing from about0.002 to 0.2% by weight nickel.
 11. The process according to claim 10,wherein the oligomerization reaction is carried out by adding the stablepreformed complex of nickel, ethylene and hydride and the bidentateligand to the oligomerization reaction zone at a molar ratio of nickelto ligand of from about 0.5:1.0 to about 5.0:1.0.
 12. The processaccording to claim 11, wherein the preformed complex of nickel, ethyleneand hydride in aliphatic diol solution and the bidentate ligand areadded to the oligomerization reaction zone to provide a concentration ofcatalyst (calculated as parts per million of ligand in the diol reactionsolvent) of at least 10 ppm.
 13. The process according to claim 12,wherein the preparation of the stable preformed complex of nickel,ethylene and hydride in diol solvent and the oligomerization reactionare carried out continuously.
 14. The process according to claim 13,wherein the amount of aliphatic diol solvent in the reaction zone rangesbetween about 0.1 to 1.0 liters of solvent per mole of ethylene.
 15. Theprocess according to claim 14, wherein the aliphatic diol solvent is1,4-butanediol, the simple divalent nickel salt is nickel chloride, theboron hydride transfer agent is sodium borohydride, the base ispotassium hydroxide and the bidentate ligand iso-diphenylphosphenobenzoic acid.
 16. The process according to claim 15,wherein the oligomerization reaction is carried out by adding the stablepreformed complex of nickel, ethylene and hydride in 1,4-butanediol andthe bidentate ligand to the oligomerization reaction zone at a molarratio of nickel to bidentate ligand of about 1.6:1.0.